stage1: always write /etc/machine-id
stage1: always write /etc/machine-id

Prepare rkt for systemd-v230 in stage1.

rkt was previously only writing /etc/machine-id in the stage1 rootfs if
systemd was running on the host. However, new versions of systemd always
require /etc/machine-id, see:
https://github.com/systemd/systemd/commit/e01ff70a77e781734e1e73a2238af2e9bf7967a8
This patch will be in systemd-v230.

This is currently not a problem because stage1 does not use systemd-v230
(and systemd-v230 is not released yet), but we want rkt to be ready.

Fixes https://github.com/coreos/rkt/issues/2416

cab105/rkt,coreos/rkt,kinvolk/rkt,jonboulle/rkt,achanda/rkt,squeed/rkt,Shashwatsh/rkt,Shashwatsh/rkt,jakon89/rkt,jakon89/rkt,mike1729/rkt,coreos/rkt,shobhit85/rkt,yifan-gu/rkt,mike1729/rkt,dgonyeo/rkt,ybubnov/rkt,ybubnov/rkt,dgonyeo/rkt,lucasallan/rkt,joshix/rkt,jakon89/rkt,jonboulle/rkt,Wilybald/rkt,euank/rkt,yifan-gu/rkt,sgotti/rkt,kinvolk/rkt,joshix/rkt,s-urbaniak/rkt,alban/rkt,Wilybald/rkt,lucasallan/rkt,jjlakis/rkt,alban/rkt,sgotti/rkt,joshix/rkt,alban/rkt,monstermunchkin/rkt,ybubnov/rkt,jonboulle/rkt,cab105/rkt,yifan-gu/rocket,tmrts/rkt,Wilybald/rkt,akshaykarle/rkt,achanda/rkt,coreos/rkt,sgotti/rkt,intelsdi-x/rkt,akshaykarle/rkt,rkt/rkt,s-urbaniak/rkt,rkt/rkt,akshaykarle/rkt,kinvolk/rkt,nhlfr/rkt,sstabellini/rkt,squeed/rkt,achanda/rkt,monstermunchkin/rkt,dgonyeo/rkt,Wilybald/rkt,cab105/rkt,euank/rkt,s-urbaniak/rkt,euank/rkt,intelsdi-x/rkt,nhlfr/rkt,yifan-gu/rkt,jjlakis/rkt,tmrts/rkt,rkt/rkt,lucasallan/rkt,yifan-gu/rocket,shobhit85/rkt,squeed/rkt,monstermunchkin/rkt,yifan-gu/rocket,lucasallan/rkt,mike1729/rkt,intelsdi-x/rkt,Shashwatsh/rkt,kinvolk/rkt,jjlakis/rkt,shobhit85/rkt,sstabellini/rkt,joshix/rkt,tmrts/rkt,sstabellini/rkt,rkt/rkt,nhlfr/rkt